ST. LOUIS — A thunderstorm complex known as a derecho moved across Missouri and Illinois during the day Thursday.
A derecho, pronounced similar to deh-REY-cho, is defined by the National Weather Service as a widespread, long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms.
While small tornadoes may be embedded within the derecho and the destruction may be similar to the strength of tornadoes, the damage typically is directed in one direction along a relatively straight swath. The word derecho means “straight ahead” in Spanish.
Meteorologists often say "straight-line wind damage" to describe derecho damage. By definition, if the wind damage swath extends more than 240 miles (about 400 kilometers) and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event may be classified as a derecho. This clearly appears to meet those criteria.
The most widespread and largest swath of damage is found throughout much of central Illinois. Nearly 200,000 power customers are waiting for crews to assess the damage and repair the lines and power poles downed by the strong winds. Winds over 80 mph were reported in the Champaign, Illinois area.
Progressive derechos are generally only warm-season events and are found moving over the north side of upper atmosphere high-pressure ridges. In meteorology, derechos move "over the top" of the ridge.
Warm-season derechos, from May through August, are responsible for 70% of all derechos. The remaining 30% happen during the "cool" season, comprising the remaining eight months.
Thursday and closer to St. Louis, most of the damage from strong winds is found in the northern portion of the 5 On Your Side viewing area. The damage stretches from Pike County, Missouri across the Mississippi River into northern Calhoun, Greene, Jersey, Macoupin, Montgomery, Fayette, Clay, and Marion counties in Illinois.
Ameren Illinois tells 5 On Your Side crews are working to restore power and that the damage is concentrated in Decatur (Macon County), Champaign (Champaign County), and Danville (Vermilion County).
The derecho produced an outflow boundary that moved through the St. Louis area around lunchtime dropping temperatures from the mid-90s into the 70s and 80s. That ended the excessive heat for Thursday afternoon in the Metro area.
With the overall weather pattern not changing much into Friday, another derecho is possible. The exact timing, location, and impacts are still to be determined.
